Heretofore, there are known a number of systems for packaging an electronic device such as TCP (tape carrier package) and COF (chip on film). The COF is paid attention because the COF is favorably employable for packaging an electronic device with an increased density.
The conventional COF is illustrated in FIG. 1. The process for packaging an electronic device according to the conventional COF system is performed by the following steps:
preparing a printed wiring board 1 which has wiring copper layers 2 coated with an insulating protective resin layer 4, keeping an area for mounting an electronic device exposed;
plating the exposed area with tin to produce a tin layer 3;
mounting the electronic device 6 on the tin-plated area via an electroconductive material 5; and
coating the mounted electronic device 6 and an edge of the insulating protective resin layer 4 with an encapsulant 7.
It is known that the insulating protective resin layer can be favorably produced by coating the wiring copper layers with a heat curable resin composition comprising an organic solvent-soluble resin having a polysiloxane skeleton (or polysiloxane chain) and a polar group, a heat curable compound and an organic solvent, and heating the coated resin composition. The heating is generally carried out at approx. 160° C.
U.S. Pat. No. 5,252,703 describes that a heat curable resin composition comprising 100 weight parts of a polyimide-siloxane, 1 to 50 weight parts of an epoxy resin, and an organic solvent can be employed for preparing an insulating protective layer on a flexible wiring board.
U.S. Pat. No. 6,461,738 describes a heat curable resin composition comprising 100 weight parts of a polyimide-siloxane, 2 to 40 weight parts of a polyvalent isocyanate, and an organic solvent can be employed for preparing an insulating protective layer on a flexible wiring board.
It has been recently noted that the conventional COF has a problem that disturbs packaging of an electronic device with a more increased density. The problem is that the plated tin enters between the wiring copper layer and the insulating protective layer and then reacts with copper in the wiring copper layer, resulting in production of a damaged portion in the wiring copper layer. The production of the damage portion in the wiring copper layer is particularly troublesome when the wiring copper layer is made thinner to further increase the packaging density.
In order to obviate the above-mentioned problem in the conventional COF packaging, an improved COF packaging system has been developed. The improved COF packaging system is described below, referring to FIG. 2.
The improved COF packaging system comprises the steps of:
preparing a printed wiring board 1 which has wiring copper layers 2 coated with a metal layer 3 comprising a metal (e.g., tin) other than copper;
plating the metal layer 3 with an insulating protective resin layer 4, keeping an area for mounting an electronic device exposed;
mounting the electronic device 6 on the exposed area via an electroconductive material 5; and
coating the mounted electronic device 6 and a portion of the insulating protective resin layer 4 with an encapsulant 7.
The improved COF packaging system is favorably employable for packaging an electronic device with a more increased density, because the electronic device can be mounted on a printed wiring board having a very thin wiring layer with no trouble of damage of the wiring copper layer.
However, there arises a new problem in that the heat curable resin composition comprising a polyimide-siloxane, a polyvalent isocyanate, and an organic solvent can be cured only at a high temperature such as approx. 160° C. and, at that temperature, tin of the plated tin layer rapidly diffuses into copper in the wiring copper layer to produce a tin-copper alloy. The tin-copper alloy is not appropriate because the tin-copper alloy formed on the exposed tin layer cannot firmly fix the electronic device via the electroconductive material (typically, gold).
There also is a problem in that the insulating protective resin layer prepared from a heat curable resin composition comprising a polyimide-siloxane, an epoxy resin, and an organic solvent has poor affinity and poor compatibility with the encapsulant.